Purification of acetic acid produced by the low water carbonylation of methanol by treatment with ozone

ABSTRACT

Acetic acid produced by the low water carbonylation of methanol and containing iodide, unsaturates and carbonyl impurities is purified by treatment with ozone.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 07/657,963,filed Feb. 20, 1991, now abandoned, which is a continuation ofapplication Ser. No. 07/447,450, filed Dec. 7, 1989, now abandoned,which is a continuation-in-part of application Ser. No. 137,844, filedDec. 23, 1987, now abandoned which is a continuation-in-part ofapplication Ser. No. 137,844, filed Dec. 23, 1987 (now abandoned).

Application Ser. No. 936,188, filed Dec. 1, 1986 (now abandoned),discloses purification of acetic acid by treatment with a compound suchas hydrazine or derivatives thereof.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the purification of acetic acid and relatesmore particularly to the purification of acetic acid resulting from thelow water catalytic , carbonylation of methanol.

2. Description of the Prior Art

Various methods have been employed for producing acetic acid including,for example, the oxidation of acetaldehyde, the oxidation of petroleumnaphtha, butane or the like, or the direct synthesis of methanol andcarbon monoxide. One of the more useful commercial methods for theproduction of acetic acid is the carbonylation of methanol as disclosedin U.S. Pat. No. 3,769,329. The carbonylation catalyst comprisesrhodium, either dissolved or otherwise dispersed in a liquid reactionmedium or else supported on an inert solid, along with ahalogen-containing catalyst promoter as exemplified by methyl iodide.The rhodium can be introduced into the reaction system in any of manyforms, and it is not relevant, if indeed it is possible, to identify theexact nature of the rhodium moiety within the active catalyst complex.Likewise, the nature of the halide promoter is not critical. A largenumber of suitable promoters are disclosed, most of which are organiciodides. Typically, the reaction is conducted with the catalyst beingdissolved in a liquid reaction medium through which carbon monoxide gasis continuously bubbled.

An improvement in the prior art process for the carbonylation of analcohol to produce the carboxylic acid having one carbon atom more thanthe alcohol in the presence of a rhodium catalyst is disclosed incopending, commonly assigned application U.S. Ser. No. 699,525, filedFeb. 8, 1985; European Patent Application No. 161,874, published Nov.21, 1985; and U.S. Ser. No. 870,267, filed Jun. 3, 1986. As disclosedtherein, acetic acid (HAc) is produced from methanol (MeOH) in areaction medium comprising methyl acetate (MeOAc), methyl halide,especially methyl iodide (MeI), and rhodium present in acatalytically-effective concentration. The invention therein residesprimarily in the discovery that catalyst stability and the productivityof the carbonylation reactor can be maintained at surprisingly highlevels, even at very low water concentrations, i.e., 4 wt. % or less, inthe reaction medium (despite the general industrial practice ofmaintaining approximately 14 wt. % or 15 wt. % water) by maintaining inthe reaction medium, along with a catalytically-effective amount ofrhodium, at least a finite concentration of water, methyl acetate andmethyl iodide, a specified concentration of iodide ions over and abovethe iodide content which is present as methyl iodide or other organiciodide. The iodide ion is present as a simple salt, with lithium iodidebeing preferred. The applications teach that the concentration of methylacetate and iodide salts are significant parameters in affecting therate of carbonylation of methanol to produce acetic acid especially atlow reactor water concentrations By using relatively highconcentrationsof the methyl acetate and iodide salt, one obtains asurprising degree of catalyst stability and reactor productivity evenwhen the liquid reaction medium contains water in concentrations as lowas about 0.1 wt. %, so low that it can broadly be defined simply as "afinite concentration" of water. Furthermore, the reaction mediumemployed improves the stability of the rhodium catalyst, i.e.,resistance to catalyst precipitation, especially during theproduct-recovery steps of the process wherein distillation for thepurpose of recovering the acetic acid product tends to remove from thecatalyst the carbon monoxide which in the environment maintained in thereaction vessel, is a ligand with stabilizing effect on the rhodium.U.S. Ser. No. 699,525 and U.S. Ser. No. 870,267 are herein incorporatedby reference.

The acetic acid which is formed by the carbonylation of methanol isconverted to a high purity product by conventional means such as by aseries of distillations. While it is possible in this manner to obtainacetic acid of relatively high purity, the acetic acid product containsa considerable amount of by-product impurities, determinable on thebasis of their reducing action on permanganate. The amount of suchreducing impurities is referred to as the permanganate time. Since thepermanganate time is an important commercial test which the acid productmust meet for many uses, the presence therein of such impurities ishighly objectionable. Apparently, the removal of minute quantities ofthese impurities by conventional rectification alone is difficult sincethe impurities distill over with the acetic acid.

Among the residual impurities which have been found to degrade thepermanganate time are alkyl iodide impurities which are most likelycarried over into the product stream from the catalyst solution in thereactor. Also found in the acetic acid product are various unsaturatedand carbonyl impurities including crotonaldehyde, ethyl crotonaldehydeand the 2-methyl-2-pentanal isomer thereof. As has been previouslystated, it is both difficult and costly to remove the iodides,unsaturates and carbonyl impurities from the acetic acid product byphysical methods since such impurities are present in such minuteamounts. Accordingly, an economical process for removing such impuritiesis needed.

Various methods have been suggested to purify or remove nonacidiccomponents from carboxylic acids. For example, U.S. Pat. No. 4,576,683discloses a method of separating C₁ -C₁₀ aliphatic and C₃ -C₁₀ olefiniccarboxylic acids from mixtures with nonacids by extractive distillationusing an amide as an extractant to recover an extractant-acid mixture byrectification. The method disclosed in the patent is described as beingparticularly suitably applied on aqueous mixtures of formic, aceticand/or propionic acid which mixtures contain unconverted hydrocarbonsand other oxygenated compounds such as mixtures with alcohols, aldehydesand/or ketones and which may also contain further contaminants such aseffluents from the tent are selected from lactams having 5 or 6 memberedrings Pyrrolidone and derivatives thereof are specifically disclosed.

U.S. Pat. No. 4,268,362 is concerned with providing a method of removingformaldehyde from raw acetic acid which has been formed by syntheticreactions such as oxidation of acetaldehyde, gas phase or liquid phaseoxidation of butane, oxidation of petroleum naphtha or paraffins, aswell as the reaction of methanol with carbon monoxide. The separationprocess involves treating the acetic acid in a heating zone at atemperature at about the boiling point of the acetic acid or higher,removing the heated product and delivering it to a distillation zone andoperating the distillation zone so as to obtain a lower boilingfraction, a higher boiling fraction and an intermediate acetic acidfraction which will have a formaldehyde content of 300 ppm or lower.

U.S. Pat. No. 3,725,208 is concerned with a process for the removal ofsmall amounts of aldehyde impurities from acrylic acids which comprisesadding to the acrylic acid minor amounts of a compound selected from thegroup consisting of sulfuric acid, hydrazine, phenyl hydrazine, aniline,monoethanolamine, ethylene diamine and gylcine and subjecting theacrylic acid mixture to distillation. Although hydrazine usually reactsexothermically with acrylic acid to form pyrrazolidone, and amines suchas monoethanolamine and ethylene diamine have the properties of formingsalts and aminocarboxylic acids with these compounds react predominantlywith aldehydes contained in acrylic acid and can remove them from theacrylic acid.

Japanese Patent Application 60-222439 discloses purification of aceticanhydride produced by the ketene process in which acetic acid isthermally cracked to ketene which then combines with acetic acid throughan absorption reaction to produce acetic anhydride. The impuritiescontained in acetic anhydride produced in this manner are many low andhigh boiling compounds present at the time when acetic acid is thermallycracked and when acetic acid and ketene are reacted. However, the exactnature of the impurities contained in the acetic anhydride are notdisclosed. Treatment with ozone gas in the absence of an oxidationcatalyst was found to provide a quality product equal to or greater thanthat produced in purification by distillation.

Japanese Patent Publication 55(1980)-64545, published May 15, 1980,discloses purification of acetic acid in which an ozone-containing gasis introduced to the acetic acid in the absence of an oxidation catalystto obtain acetic acid of a higher quality as measured by a potassiumpermanganate test and sulfuric acid colon test. The identities of theimpurities contained in acetic acid are not identified.

U.S. Pat. No. 3,928,434 to Saunby discloses reducing the content ofoxidizable impurities in acetic acid produced by hydrocarbon oxidationby treating the acetic acid with oxygen in the presence of a transitionmetal compound to oxidize unsaturated ketones. Saunby, in col. 1, lines45-60, points out that, heretofore, oxidizable impurities can bedestroyed by reaction with ozone, but that such treatment suffers thedrawback of the risk involved in handling ozone at elevated temperaturesin organic liquid. The Saunby disclosure is directed to removal ofalpha, beta-unsaturated ketone impurities in acetic acid produced byhydrocarbon oxidation.

SUMMARY OF THE INVENTION

The present invention is directed to the purification of acetic acid andthe improvement of permanganate time by subjecting the acid to treatmentwith ozone or an ozone containing gas. Thus acetic acid, as formed andrecovered from the low water catalytic carbonylation of methanol, can bepurified of minute amounts of unsaturates, iodides and carbonylcompounds by treatment with ozone which reacts with such impurities. Theozone-derived impurities are subsequently separated from the acetic acidby adsorption on an absorbent such as activated carbon, or anion-exchange resin which is at least partially converted to the silveror mercury form.

DETAILED DESCRIPTION OF THE INVENTION

The ozonolysis treatment of the present invention is applicable to thepurification of acetic acid which has been produced by the low watercarbonylation of methanol in the presence of a metal catalyst such asrhodium. The purification process of the present invention isparticularly useful when the carbonylation reaction is catalyzed by ametal such as rhodium and a halide promoter such as an organic halidedisclosed in U.S. Pat. No. 3,769,329 to Paulik et al. The process ofpurifying acetic acid in the present invention is more particularlyuseful when the acetic acid is formed by the carbonylation of methanolunder low water conditions such as set out in U.S. Ser. No. 699,525wherein the catalyst solution not only contains the rhodium catalyst andorganic halide promoter, but also contains an additional iodide salt. Ithas been found that organic iodide impurities as well as unsaturated andcarbonyl impurities degrade the commercial value of the acetic acidproduct.

In the low water carbonylation of methanol to acetic acid as exemplifiedin U.S. Ser. No. 699,525 and U.S. Ser. No. 870,267, the catalyst whichis employed includes a rhodium component and a halogen promoter in whichthe halogen is either bromine or iodine. Generally, the rhodiumcomponent of the catalyst system is believed to be present in the formof a coordination compound of rhodium with a halogen component providingat least one of the ligands of such coordination compound. In additionto the coordination of rhodium and halogen, it is also believed thatcarbon monoxide ligands form coordination compounds or complexes withrhodium. The rhodium component of the catalyst system may be provided byintroducing into the reaction zone rhodium in the form of rhodium metal,rhodium salts and oxides, organic rhodium compounds, coordinationcompounds of rhodium, and the like.

The halogen promoting component of the catalyst system consists of ahalogen compound comprising an organic halide. Thus, alkyl, aryl, andsubstituted alkyl or aryl halides can be used. Preferably, the halidepromoter is present in the form of an alkyl halide in which the alkylradical corresponds to the alkyl radical of the feed alcohol which iscarbonylated. For example, in the carbonylation of methanol to aceticacid, the halide promoter will comprise methyl halide, and morepreferably methyl iodide.

The liquid reaction medium employed may include any solvent compatiblewith the catalyst system and may include pure alcohols, or mixtures ofthe alcohol feedstock and/or the desired carboxylic acid and/or estersof these two compounds. The preferred solvent and liquid reaction mediumfor the low water carbonylation process comprises the carboxylic acidproduct. Thus, in the carbonylation of methanol to acetic acid, thepreferred solvent is acetic acid.

Water is also added to the reaction medium but at concentrations wellbelow what has heretofore been thought practical for achievingsufficient reaction rates. It is known that in rhodium-catalyzedcarbonylation reactions of the type set forth in this invention, theaddition of water exerts a beneficial effect upon the reaction rate,U.S. Pat. No. 3,769,329 to Paulik. Thus, commercial operations run atwater concentrations of at least 14 wt. %. Accordingly, it is quiteunexpected that reaction rates substantially equal to and above reactionrates obtained with such high levels of water concentration can beachieved with water concentrations below 14 wt. % and as low as 4.0 wt.% to 0.1 wt. %.

In accordance with the carbonylation process most useful in the presentinvention, the desired reaction rates are obtained even at low waterconcentrations by including in the reaction medium an ester whichcorresponds to the alcohol being carbonylated and the acid product ofthe carbonylation reaction and an additional iodide ion which is overand above the iodide which is present as a catalyst promoter such asmethyl iodide or other organic iodide. Thus, in the carbonylation ofmethanol to acetic acid, the ester is methyl acetate and the additionaliodide promoter is an iodide salt, with lithium iodide being preferred.It has been found that under low water concentrations, methyl acetateand lithium iodide act as rate promoters and catalyst stabilizers onlywhen relatively high concentrations of 5 wt. % to 20 wt. % of each ofthese components are present and that the promotion is higher when bothof these components are present simultaneously. This has not beenrecognized in the prior art previous to disclosure of commonly assignedU.S. Ser. No. 699,525 and U.S. Ser. No. 870,267. The concentration oflithium iodide used in the reaction medium of the preferredcarbonylation reaction system is believed to be quite high as comparedwith what little prior art there is dealing with the use of halide saltsin reaction systems of this sort.

The carbonylation reaction may be carried out by intimately contactingthe feed alcohol, which is in the liquid phase, with gaseous carbonmonoxide bubbled through a liquid reaction medium containing the rhodiumcatalyst, halogen-containing promoting component, alkyl ester, andadditional soluble iodide salt promoter, at conditions of temperatureand pressure suitable to form the carbonylation product. Thus, when thefeed is methanol, the halogen-containing promoting component willcomprise methyl iodide and the alkyl ester will comprise methyl acetate.It will be generally recognized that it is the concentration of iodideion in the catalyst system that is important and not the cationassociated with the iodide, and that at a given molar concentration ofiodide, the nature of the cation is not as significant as the effect ofthe iodide concentration. Any metal iodide salt, or any iodide salt ofany organic cation, can be used provided that the salt is sufficientlysoluble in the reaction medium to provide the desired level of theiodide. The iodide salt can be a quaternary salt of an organic cation orthe iodide salt of an inorganic cation. Preferably, it is an iodide saltof a member of the group consisting of the metals of Group Ia and GroupIIa of the Periodic Table as set forth in the "Handbook of Chemistry andPhysics" published by CRC Press, Cleveland, Ohio, 1975-76 (56thEdition). In particular, alkali metal iodides are useful, with lithiumiodide being preferred. In the low water carbonylation most useful inthis invention, the additional iodide over and above the organic iodidepromoter is present in the catalyst solution in amounts of from 2-20,preferably 10-20 wt. %, the methyl acetate is present in amounts of from0.5-30, preferably 2-5 wt. %, and the methyl iodide is present inamounts of from 5-20 and 14-16 wt. %. The rhodium catalyst is present inamounts of from 200-1,000 and preferably 300-600 ppm.

Typical reaction temperatures for carbonylation will be approximately150°-250° C., with the temperature range of about 180°-220° C. being thepreferred range. The carbon monoxide partial pressure in the reactor canvary widely but is typically about 2-30 atmospheres and preferably about4-15 atmospheres. Because of the partial pressure of by-products and thevapor pressure of the contained liquids, the total reactor pressure willrange from about 15-40 atmospheres.

A reaction and acetic acid recovery system which can be employed, withinwhich the present improvement is used, comprises (a) a liquid-phasecarbonylation reactor, (b) a so-called "flasher", and (c) a "methyliodide-acetic acid splitter column". The carbonylation reactor istypically a stirred autoclave within which the reacting liquid contentsare maintained automatically at a constant level. Into this reactorthere are continuously introduced fresh methanol, sufficient water tomaintain at least a finite concentration of water in the reactionmedium, recycled catalyst solution from the flasher base, and recycledmethyl iodide and methyl acetate from the overhead of the methyliodide-acetic acid splitter column. Alternate distillation systems canbe employed so long as they provide means for recovering the crudeacetic acid and recycling to the reactor catalyst solution, methyliodide, and methyl acetate. In the preferred process, carbon monoxide iscontinuously introduced into the carbonylation reactor just below theagitator which is used to stir the contents. The gaseous feed is, ofcourse, thoroughly dispersed through the reacting liquid by this means.A gaseous purge stream is vented from the reactor to prevent buildup ofgaseous by-products and to maintain a set carbon monoxide partialpressure at a given total reactor pressure. The temperature of thereactor is controlled automatically, and the carbon monoxide feed isintroduced at a rate sufficient to maintain the desired total reactorpressure.

Liquid product is drawn off from the carbonylation reactor at a ratesufficient to maintain a constant level therein and is introduced to theflasher at a point intermediate between the top and bottom thereof. Inthe flasher, the catalyst solution is withdrawn as a base stream(predominantly acetic acid containing the rhodium and the iodide saltalong with lesser quantities of methyl acetate, methyl iodide andwater), while the overhead of the flasher comprises largely the productacetic acid along with methyl iodide, methyl acetate and water. Aportion of the carbon monoxide along with gaseous by-products such asmethane, hydrogen and carbon dioxide exits the top of the flasher.

The product acetic acid drawn from the base of the methyl iodide-aceticacid splitter column (it can also be withdrawn as a side stream near thebase) is then drawn off for final purification such as to remove wateras desired by methods which are obvious to those skilled in the artincluding, most preferably, distillation. The overhead from the methyliodide-acetic acid splitter, comprising mainly methyl iodide and methylacetate, is recycled to the carbonylation reactor along with freshmethyl iodide, the fresh methyl iodide being introduced at a ratesufficient to maintain in the carbonylation reactor the desiredconcentration of methyl iodide in the liquid reaction medium. The freshmethyl iodide is needed to compensate for losses of methyl iodide in theflasher and carbonylation reactor vent streams.

The crude dry acetic acid product is not adequately purified since itcontains residual by-products such as organic and metal iodides,unsaturates, and carbonyl impurities of which crotonaldehyde, ethylcrotonaldehyde and 2-methyl-2-pentanal are the most prominent. In thehigh water carbonylation of methanol of Paulik et al (U.S. Pat. No.3,769,329), which generally teaches that a substantial quantity of waterhelps in obtaining adequately high reaction rates, and in EuropeanPatent Application 0055618, which teaches that typically 14-15 wt. %water is in the reaction medium of a typical acetic acid plant usingthis technology, impurities such as 2-methyl-2-pentanal, crotonaldehydeand ethyl-crotonaldehyde are not present but become a problem as thewater content is lowered below 14 wt. % in the low water carbonylationof methanol. Small amounts of these impurities degrade the commercialusefulness of the acetic acid product and, accordingly, it has beendiscovered that by treating the acetic acid with ozone, it becomespossible to obtain a desired degree of purification as evidenced by thepermanganate test.

According to the invention, the crude acetic acid is subject toozonolysis by generating the ozone gas and bringing the gas intophysical contact with the acetic acid product in the presence of acatalytically effective amount of an oxidation catalyst.

Ozone (O₃) is a gaseous allotropic form of oxygen in which three atomsform the molecule rather than the normal two. Although ozone is a strongoxidizing agent, it is not a specific oxidant and, hence, will oxidizeany material it contacts which has a lower oxidation potential. As such,when it contacts the aforesaid impurities in acetic acid, it willoxidize the carbon to carbon double bond linkages of unsaturates, forexample, which apparently contribute to short permanganate times in theacetic acid product. This theory of operation, however, is not to beregarded as essential to an understanding of the invention. Availabledata, as shown hereinafter, indicate that the benefit of the ozonecontacting is due to the ability of the ozone to render iodides,unsaturates and carbonyl compounds inactive, thus preventing theirinfluence on acetic acid either during storage or subsequent use.

Ozonolysis may be carried out by generating the ozone from any suitablesource such as a quartz lamp, a silent electric discharge or sparkdischarge commonly called corona discharge, but it is preferable toobtain the ozone from a source of radiation in the range between about1000 and 2950 angstrom units in wave length, applied in air or oxygen.For commercial production of ozone, it is preferable to use coronadischarge technology on either air or oxygen. UV radiation typegenerators are usually only used on a small scale system. The maximumweight ratio of ozone in the liquid acetic acid is governed by theflammability limits of acetic acid--O₂ vapor phase compositions. In theexamples hereinafter set forth, ozone was introduced into the mid pointof a cylindrical vessel and contacted with a downwardly flowing streamof acetic acid at a temperature of about 95° F. (35° C.). Sufficientpressure was employed to keep the acetic acid below the flammabilitylimit of 2.5 volume percent in oxygen or 3.8 volume percent in air (˜10psig in air or 25 psig in O₂).

The ozone exposure time will vary, but it has been found that the effectis substantially instantaneous, while on the other hand, over exposureis not harmful. Good results are obtained when the exposure time is lessthan one-half hour, usually about 1 to 15 minutes. The preferredquantity of ozone will range from about 3 ppm to 5000 ppm based on theweight of the acetic acid treated. High levels of ozone are notdetrimental except for associated costs.

The ozonolysis may be carried out at temperatures of 70° F. (21° C.) to115° F. (52° C.) in a continuous or batchwise fashion. Temperature andpressure considerations are not critical so long as flammability limitsare not exceeded.

The iodides, unsaturates and carbonyl impurities in acetic acidapparently react with ozone to form a reactive oxygenated species orcomplex which may be separated from the acetic acid. Such separation canbe accomplished by passing the solution through a carbonaceous materialor a macroreticulated strong-acid cation exchange resin which is stablein the organic medium and has at least one percent of its active sitesconverted to the silver or mercury ion-exchange form.

As indicated, the ion exchange resin has been at least partiallyconverted to the silver or mercury form. It is important in practice touse an ion exchange resin with suitable properties. The ion exchangeresin should not be of the gel-type. As is known, gel-type polymers arecharacterized by the fact that their porosity essentially depends on thevolume increase which they exhibit upon exposure to a given solventsystem. Ion exchange resins which depend essentially upon swelling fortheir porosity are not suitable for the practice of the presentinvention.

The ion exchange resins used in the present invention may thus be termed"non-gel-type" ion exchange resins. Such useful resins are typicallyconsidered to be macroreticular ion exchange resins and usually havepores considerably larger than those of the gel-type. However, thepresent invention is not limited to any specific pore-size of theion-exchange resin. Usually the ion exchange resins used in the presentinvention have an average pore size from about 50 to 1,000 angstroms.Preferably, the average pore size is from about 200 to 700 angstroms.

The ion-exchange resin should also be of the type typically classifiedas a "strong acid" cation exchange resin. Preferably the resin of the"RSO₃ H type." It is beyond the scope of the present invention to teachhow to manufacture or otherwise characterize ion exchange resins, assuch knowledge is already well known in that art. For the purposes ofthe present invention it is sufficient to characterise an ion exchangeresin useful therein as being a strongly-acidic cation exchange resin ofthe non-gel type, and thus macroreticulated.

A preferred ion exchange resin for use in the practice of the presentinvention is a macroreticulated resin comprised a sulfonated copolymerof styrene and divinyl benzene. The most preferred resin such as thatavailable from Rohm and Haas under the trademark Amberlyst® 15, has thefollowing properties:

    ______________________________________                                                               Hard, dry                                                                     spherical                                              Appearance             particles                                              ______________________________________                                        Typical particle size distribution                                            percent retained on                                                           16 mesh U.S. Standard Screens                                                                        2-5                                                    -16 + 20 mesh U.S. Standard Screens                                                                  20-30                                                  -20 + 30 mesh U.S. Standard Screens                                                                  45-55                                                  -30 + 40 mesh U.S. Standard Screens                                                                  15-25                                                  -40 + 50 mesh U.S. Standard Screens                                                                   5-10                                                  Through 50 mesh, percent                                                                             1.0                                                    Bulk density, lbs./cu. ft.                                                                           38 (608 g/L)                                           Moisture, by weight    less than 1%                                           Percentage swelling from dry state                                            to solvent-saturated state                                                    hexene                 10-15                                                  toluene                10-15                                                  ethylene dichloride    15-20                                                  ethyl acetate          30-40                                                  ethyl alcohol (95%)    60-70                                                  water                  60-70                                                  Hydrogen ion concentration                                                                           4.7                                                    meq./g. dry                                                                   Surface Area, m.sup.2 /g.                                                                            50                                                     Porosity, ml. pore/ml. bead                                                                          0.36                                                   Average Pore Diameter, Angstroms                                                                     240                                                    ______________________________________                                    

A final characteristic of the resin when used to remove iodide compoundsfrom non-aqueous, organic media, and one that is inherent in most ionexchange resins meeting the foregoing requirements, especially when theresin is specifically indicated to be designated for non-aqueousapplications, is that the resin is stable in the organic medium fromwhich the iodide compounds are to be removed. By the term "stable," itis meant that the resin will not chemically decompose, or change morethan about 50 percent of its dry physical dimension upon being exposedto the organic medium containing the iodide compounds.

The ion exchange resin as indicated above, should be at least partiallyconverted to the silver or mercury form. Conversion to the silver formis preferred.

The method of converting the ion exchange resin to the silver or mercuryform is not critical. Any mercury or silver salt which has reasonablesolubility in water or a suitable non-aqueous organic medium can beused. Silver acetate and silver nitrate are typical salts. The organicmedium which may be used to load silver ions on the exchange resin maybe, for example, acetic acid. When mercury is desired, rather thansilver, a suitable salt is mercuric acetate.

The ion exchange resin is converted, to the desired degree, to thesilver or mercury form, by simply contacting the resin with a solutionof the desired silver or mercury salt for a sufficient length of time toallow for association of the metal ions with the resin.

The amount of silver or mercury associated with the resin is notcritical and may be from as low as about 1 percent of the active acidsites to as high as 100 percent, converted to the silver or mercuryform. Preferably about 25 percent to about 75 percent are converted tothe silver or mercury form, and most preferably about 50 percent. Asstated previously, the preferred metal is silver.

As some silver may be leached from the silver-treated ion exchange resinduring conditions of actual use, it may be useful to have a bed ofion-exchange resin which has not been bed of silver-treated ion exchangeresin. With respect to the processing steps, the non-aqueous organicmedium which contains the iodide impurities is simply placed in contactwith the silver-loaded ion exchange resin described above, using anysuitable means. For example, the resin may be packed into a column bypouring slurries thereof into a column. The organic medium is thensimply allowed to flow therethrough. Any other suitable means of placingthe resin in contact with the organic medium may be employed.

When a packed column is used, the organic medium is usually allowed toflow therethrough at a predetermined rate. The particular rate used inany given instance will vary depending upon the properties of theorganic medium, the particular resin, the degree and nature of theiodide compounds to be removed, and the percent of iodide compounds tobe removed.

A typical flow rate, such as is used when acetic acid is to be purified,is from about 0.5 to about 20 bed volumes per hour ("BV/hr"). A bedvolume is simply the volume of the resin bed. A flow rate of 1 BV/hrthen means that a quantity of organic medium equal to the volumeoccupied by the resin bed passes through the resin bed in a one hourtime period. Preferred flow rates are usually about 6 to about 10 BV/hrand the most preferred flow rate is usually about 8 BV/hr.

The temperature at which the iodide compound removal takes place is alsonot critical. Broadly, the method may be performed at any temperaturefrom about the freezing point of the organic liquid to the decompositiontemperature of the resin. As a practical matter, the temperatureemployed is usually from about 17° C. to about 100° C., typically fromabout 18° C. to about 50° C., and preferably under ambient conditions ofabout 20° C. to about 45° C.

In one embodiment of the present invention the non-aqueous organicmedium is contacted with a carbonaceous material in addition tocontacting the aforementioned ion exchange resin. Preferably, thecarbonaceous material is used in a contacting step prior to the step ofcontacting the ion exchange resin. Although the aforementioned ionexchange resin is useful in removing iodide compounds, it is not veryeffective in removing iodine itself.

As discussed in U.S Pat. No. 1,843,354, carbonaceous materials have beenfound to be effective absorbers of iodine. Carbonaceous materials listedtherein include activated carbons, wood charcoals, bone char, ligniteand the like. Preferably, activated carbon is used. It appears thatactivated carbons of the type usually identified as gas-phase carbonswork best in removing iodine from such organics. Gas-phase activatedcarbons typically have surface areas on the order of 1,000 to 2,000 m²/g. The most preferred activated carbon is one derived from coconutshells, such as is available under the designation Pittsburgh PCB 12X30carbon.

Usually the non-aqueous organic medium is placed in contact with thecarbonaceous material in the same manner as with the ion exchange resin,under the same or comparable conditions.

The present invention can be more fully understood by referring to thefollowing examples which illustrate the best mode now contemplated forcarrying out of the invention. In the examples the "permanganate time"is determined as follows:

One ml of an aqueous 0.1N potassium permanganate solution is added to 50ml of acetic acid in a graduated cylinder at room temperature. Thecylinder is stoppered and shaken, and a timer is immediately started tomeasure the time required for the purple color to change to ayellow-amber end point which is compared to a standard reference colorindicating the content of unsaturate, iodide and carbonyl impurities.

EXAMPLE 1

185 grams of glacial finished acetic acid [obtained from a low water(<14 wt. %) carbonylation of methanol to acetic acid employing a halogenpromoted rhodium catalyst] spiked to contain 231 ppm crotonaldehyde and224 ppm ethyl crotonaldehyde impurities and having a permanganate timeof 0.1 minutes were treated for 30 minutes with ozone made from air (0.5vol. % ozone in air).

EXAMPLE 2

Unspiked glacial finished acetic acid [obtained from a low water (<14wt. %) carbonylation of methanol to acetic acid employing a halogenpromoted rhodium catalyst] was treated with ozone in the manner ofExample 1 in which the contact time with ozone was 10 and 19 minutes.

EXAMPLE 3

Glacial finished acetic acid [obtained from a low water (<14 wt. %)carbonylation of methanol to acetic acid employing a halogen promotedrhodium catalyst] was spiked with ethyl crotonaldehyde (23.2 ppm) andtreated with ozone in the manner of Example 1.

EXAMPLE 4

A glacial finished acetic acid overhead cut identified as T-840H[obtained from a low water (<14 wt. %) carbonylation of methanol toacetic acid employing a halogen promoted rhodium catalyst] was treatedwith ozone and compared to an untreated sample.

EXAMPLE 5

Two samples of glacial acetic acid, identified as V-783 outlet [obtainedfrom a low water (<14 wt. %) carbonylation of methanol to acetic acidemploying a halogen promoted rhodium catalyst], were treated with ozoneat different contact times of 1 and 3 minutes and compared to anuntreated sample.

EXAMPLE 6

A sample of the untreated glacial acetic acid used in Example 5 wastreated with ozone in the presence of 0.5 wt. % manganese dioxidecatalyst in two different runs and compared to a sample that was treatedwith ozone in the absence of a catalyst and a sample that was treatedwith 0.5% manganese dioxide and air.

The test results of Examples 1 to 6 and the parameters of the tests areset forth below in Table 1.

                                      TABLE 1                                     __________________________________________________________________________                                      Feed O.sub.3                                                             O.sub.3                                                                            Concen.                                                                            O.sub.3                                                                            O.sub.3                                                                             Total                                                                             Total                                                                             Wt. of                             Et  KMnO.sub.4                                                                         Peroxide                                                                           Contact                                                                            Based.                                                                             Concen.                                                                            Gener.                                                                              Gas O.sub.3                                                                           HAC                            Crot.                                                                             Crot.                                                                             Time Concen.                                                                            Time on HAC                                                                             in air                                                                             Rate  Cont.                                                                             Cont.                                                                             Treated             Ex.                                                                              HAC     (ppm)                                                                             (ppm)                                                                             (min.)                                                                             (ppm)                                                                              (min.)                                                                             (ppm)                                                                              (%)  (mg/min.)                                                                           ( ) (mg)                                                                              (gm)                __________________________________________________________________________    1  Spiked HAC                                                                            231 224 0.1  --   --   --   --   --    --  --  185.0                  blank*                                                                        Spiked HAC +                                                                          9   3   0.25 --   30   --   --   --    --  --  185.0                  O.sub.3 *                                                                  2  HAC blank                                                                             5.2 5.0 1.5  --   --    0   --   --    0   0   183.2                  HAC + O.sub.3                                                                         0.1 0.6 --   --   10   42   0.09 0.77  4.0 7.7 183.2                  (10 min.)                                                                     HAC + O.sub. 3                                                                        0.1 0.5 5.0  <3   19   88   0.09 0.82  8.1 15.6                                                                              177.9                  (19 min.)                                                                  3  Spiked HAC**                                                                          6.2 23.2                                                                              0.25 --   --    0   --   --    0   0   198.9                  Spiked HAC                                                                            0.1 0.1 --   --   2    56   0.6  5.5   0.86                                                                              11.1                                                                              198.9                  2 min O.sub.3                                                                 Spiked HAC                                                                            0.1 0.5 --   --   4    114  0.6  5.5   1.72                                                                              22.1                                                                              193.7                  4 min O.sub.3                                                                 Spiked HAC                                                                            0.1 0.3 1.0  <3   5    146  0.6  5.5   2.15                                                                              27.6                                                                              188.4                  5 min O.sub.3                                                              4  T-8404H blank                                                                         6.4 5.2 1.0  --   --   --   --   --    --  --  --                     T-8404H + O.sub.3                                                                     0.1 0.1 8    --   1.0  22   0.60 4.1   0.32                                                                              4.1 190.0               5  V-783 blank                                                                           6.2 7.1 0.75 --   --   --   --   --    --  --  --                     V-783 + O.sub.3                                                                       .1  .1  3    --   1.0  34   0.4  6.2   0.73                                                                              6.2 182.22                 V-783 + O.sub.3                                                                       .1  .1  3    --   3.0  26   0.1  1.6   2.05                                                                              4.9 191.22              6  V-783 + O.sub.3 +                                                                     .1  .1  4    --   1.0   6   .07  1.1   0.74                                                                              1.1 188.91                 MnO.sub.2                                                                     V-783 + O.sub.3                                                                       .1  .1  3.25 --   6    36   .05  1.1   2.5 6.8 190.21                 V-783 + O.sub.3 +                                                                     .1  .1  12   --   6    36   .05  1.1   2.5 6.8 187.60                 MnO.sub.2                                                                     V-783 + air +                                                                         6.0 7.3 0.75 --   --   --   0    0     2.5 0   185.0                  MnOz.sub.2                                                                 __________________________________________________________________________     *Spiked with Crotonaldehyde and Ethyl Crotonaldehyde                          **Spiked with Ethyl Crotonaldehyde.                                      

EXAMPLES 7 to 12

In the following examples, improvement in permanganate time is furtherachieved by ozonolysis followed by activated carbon treatment. Aone-half gallon sample of ozone treated acetic acid, as obtained inExample 1, was passed through activated carbon in a flooded or tricklebed system at ambient room temperature and pressure. The results areshown in Table 2 below.

                  TABLE 2                                                         ______________________________________                                                              Permanganate                                            Example               Time, Min                                               ______________________________________                                         7.    untreated glacial acetic acid***                                                                 0.25                                                 8.    ozone treated acid 1.5                                                  9.    ozone treated acid & activated                                                                   2.5                                                        carbon* (Flooded Bed)                                                  10.    ozone treated acid & activated                                                                   4.5                                                        carbon* (Trickle Bed)                                                  11.    ozone treated acid & activated                                                                   2.0                                                        carbon* (Flooded contactor)                                            12.    ozone treated acid & activated                                                                   10.0                                                       carbon** (Trickle Bed)                                                 ______________________________________                                         *Calgon F300                                                                  **Cocoanut Charcoal                                                           ***obtained from a low water (<14 wt. %) carbonylation of methanol to         acetic acid employing a halogen promoted rhodium catalyst                

What is claimed:
 1. A method for improving the permanganate time ofacetic acid produced by the low water carbonylation of methanol in areaction medium comprising methanol, carbon monoxide, from 0.5 to 30 wt.% methyl acetate, from 5 to 20 wt. % methyl iodide, from 2 to 20 wt. %soluble alkali metal iodide, and a halogen-promoted rhodium catalyst inthe presence of less than 14 wt. % water which comprises contacting saidacid with ozone for a period of time sufficient to provide an aceticacid product having an improved permanganate time.
 2. The method ofclaim 1 wherein the quantity of ozone contacted is greater than about 3ppm based on the weight of acetic acid tested.
 3. The method of claim 1wherein said acetic acid product is further contacted with activatedcarbon for removal of iodides, unsaturates and carbonyl impurities.